1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor integrated circuit device having a circuit formed of bipolar transistors therein.
2. Description of the Related Art
Recently, bipolar transistors whose emitter electrodes are formed of polysilicon in order to attain low power consumption high integration density and high performance have been used in a semiconductor integrated circuit device having a circuit formed of bipolar transistors. It is known that the base current of the bipolar transistor having an emitter electrode formed of polysilicon is small, and, therefore, the base Gummel number can be made large and a smaller base resistance can be obtained. As a result, particularly, the switching characteristic of the bipolar transistor can be enhanced. Further, since the emitter electrode is led out to the exterior via a polysilicon layer, it becomes possible to form a bipolar transistor having an area smaller than that of a bipolar transistor having the minimum area determined by the limitation of the photolithography including the mask alignment margin for formation of a metal layer for wirings.
However, the bipolar transistor having the emitter electrode formed of polysilicon has a defect that the characteristic of each transistor may tend to significantly vary. For example, assuming that two bipolar transistors of the above type are used as differential input transistors of differential amplifier to constitute a circuit, then a difference between the base-emitter voltages V.sub.BE of the two bipolar transistors becomes excessively large, deteriorating the reliability of the differential amplifier. That is, when a circuit has bipolar transistors used in a paired form, and the emitters or collectors thereof are connected to each other, the matching degree is lowered because of possible difference between the base emitter voltage V.sub.BE.
If variation in the base-emitter voltage VBE of the bipolar transistor having the emitter electrode formed of polysilicon is made small i order to enhance the matching degree, it becomes necessary to increase the emitter area. As a result, the area of the bipolar transistor becomes larger than that of a bipolar transistor in which the emitter electrode is not formed of polysilicon.